Brake Troubles

[mwaldrp]

I have been having trouble lately with the brakes on my '97 Jimmy. When I press the pedal, I hear a hissing noise from under the dash until the pedal is about half way down. Up to this point the brakes do not engage but they work normally if pressed farther. If I press the brakes at idle the hiss also occurs until the pedal is about half way down, at which point the hiss stops and the tachometer reading increases by about 500 rpm. Any suggestions on how to begin diagnosing this?

[Stealthee]

Probably something to do with the brake booster.

[bellaCuba]

+1^ I agree with Stealthee.

RPM rise when pumping the brakes, while idling, is normal for Chevy & GMCs, at least.

1 test you could do (even better if you know your vehicle well):
Start with the engine OFF. Pump the brake pedal until firm. Also, (1) take note how many pumps it took for it to get firm.
Next, keep your foot on the brake pedal with good pressure, then start the vehicle. (2) Pedal should drop down slightly if booster is working.

1. Should have at least 2 pumps before getting a hard pedal. If not & hard after 1 or 2 pumps, booster is not holding vacuum. Booster is bad.

2.If pedal doesn't move, then booster is bad.

If in doubt you can always try replacing the check valve 1st (in engine compartment near Master Cylinder)... if you're still having issues & failed above tests, then it's likely needing a booster.

Good luck! Keep us posted on your findings!

[mwaldrp]

Originally Posted by bellaCuba

+1^ I agree with Stealthee.

RPM rise when pumping the brakes, while idling, is normal for Chevy & GMCs, at least.

1 test you could do (even better if you know your vehicle well):
Start with the engine OFF. Pump the brake pedal until firm. Also, (1) take note how many pumps it took for it to get firm.
Next, keep your foot on the brake pedal with good pressure, then start the vehicle. (2) Pedal should drop down slightly if booster is working.

1. Should have at least 2 pumps before getting a hard pedal. If not & hard after 1 or 2 pumps, booster is not holding vacuum. Booster is bad.

2.If pedal doesn't move, then booster is bad.

If in doubt you can always try replacing the check valve 1st (in engine compartment near Master Cylinder)... if you're still having issues & failed above tests, then it's likely needing a booster.

Good luck! Keep us posted on your findings!

Thanks for the advice. I'll run this test this afternoon but I have a feeling already that I'll just have to replace the booster. Are aftermarket power brake boosters generally okay (Cardone, etc.) or is it best to spring for the original from AC Delco?

[mwaldrp]

Sorry for the slow update. I replaced the brake booster (those bolts through the firewall sure were fun!) and my braking is now as good as new. Thanks for the advice.